1. Field of the Invention
The present invention relates to a polyester film, in particular, a high strength polyester film having a coating layer on at least one surface thereof and an improved adhesion property.
2. Description of the Related Art
Polyester films, a typical example of which is a polyethylene terephthalate film, have good physical and chemical properties and are used as a graphic art material, a display material, a packaging film, and the like. Further the polyester film is widely used as a base film of a magnetic recording medium or a dielectric material of a capacitor.
Recently, in order to prolong a recording time of a magnetic tape, a film which is thinner and has higher strength is required in the field of a base film of a magnetic recording medium. The high strength film can be produced by restretching a biaxially oriented film as disclosed in Japanese Patent Publication No. 5887/196. However, since a surface of the high strength film is highly crystalline oriented, its surface has large cohesiveness so that it has poor adhesion with a magnetic paint, an adhesive, an ink, and the like.
One of the recently employed methods for improving the adhesion property of such film comprises coating a surface of the polyester film with a certain material in the production process to form a coating layer with adhesive properties as disclosed in Japanese Patent Publication No. 8470/1966. For example, an aqueous or solvent base coating liquid is applied on an unoriented or uniaxially oriented film, and the coated film is uniaxially or biaxially oriented.
Since roll stretching is usually employed in the restretching step for increasing the strength, the coating layer adheres to the heating roll so that the film is broken frequently. Therefore, the coated film is not stably produced.